Flying spot scanner color printer with color correction



y 2, 1953 G. 5. J. ALLEN ETAL 3,096,394

FLYING SPOT SCANNER COLOR PRINTER WITH COLOR CORRECTION Filed Jan. 30, 1961 2 Sheets-Sheet 1 l 8R :85 80 /o a mfi HIE mm P? 24 Mm Mm ELM/(Mi azoz/mvo mew/r mew/r MIXER Attorneys July 2, 1963 e. 5. J. ALLEN ET 3,096,394

' FLYING SPOT SCANNER COLOR PRINTER WITH COLOR CORRECTION Filed Jan. 50, 1961 2 Sheets-Sheet Z 95 EM ML i INVERTZA P? J MIXER ELM/0N6 1/30 [mu/r 54 MIA [R 1 6.? MAX IMUM SELEUOR 66 74 75 W BMW/V6 Q} Attorney:

United States atent Bee 3,996,394 FLYING SPOT SCANNER COLOR PRINTER WiTH COLOR CORRECTION Gordon Stanley James Allen, London, David Harry Mawby, Tackleway, Hastings, and Donald Charles Gresham, London, England, assignors to Crosfield Electronics Limited, a British company Filed Jan. 30, 1961, Ser. No. 85,855 Claims priority, application Great Britain Feb. 3, 1960 5 Claims. (Cl. 178-53) This invention relates to the reproduction of coloured originals and in particular to the correction of colours in the reproduction.

It is known that standard printing inks contain colour impurities so that, for example, magenta ink appears to contain some yellow. In single-stage masking a correction is made for this impurity by reducing the yellow printer density (whenever there is sufiicient yellow to enable this to be done) in proportion to the magneta printer density. However, this does not provide good colour correction, firstly because of the failure of the effective blue-filter density of the superimposed magenta and yellow inks to equal the sum of the blue-filter densities of the inks when printed side by side, and secondly because the uncorrected negatives are matched to a neutral scale and too great a reduction of the yellow printer by a correcting printer will introduce serious colour distortion into the neutral tones. Such correction also reduces the contrast of the grey scale, and this is particularly apparent when one colour channel signal (for example the yellow printer signal) is corrected from both the cyan printer and the magenta printer channels. The degree of correction obtained by this method is therefore usually limited by the amount of grey scale compression which is permissible.

The reduction of the grey scale can be prevented by subtracting the signal to be corrected from the signal in the correcting channel so that the final correcting signal is zero when the two colours are present in equal density. Furthermore by distorting the waveform of the difference signal by means of a non-linear circuit some degree of compensation for the 'additivity failure can be obtained. However, in order to obtain a high degree of compensation the gain in the circuits from which the correcting signal is derived must be high and this has the disadvantage that minor blemishes in the original are sometimes magnified in the reproduction.

According to the present invention, a set of electric colour channel signals which vary with the col-our components of successive elements of a coloured original is obtained by means of an electro-optical scanner, and a colour channel signal representing the colour to be corrected is subtracted from a colour channel signal corresponding to a given correcting colour, the difference signal being then passed through a non-linear circuit which attenuates at least the difference signals of a polarity representing an excess of the signal to be corrected over the correcting colour, a light-sensitive layer which is to provide a corrected colour printer being exposed in accordance with the colour-component values to be corrected modified in accordance With both the signal from the nonlinear circuit and the colour channel signal for the said correcting colour. If there is more than one correcting col-our channel, the same method can be used for each correcting channel and the resultant signals may be combined in a mixing circuit. An uncorrected black printer having densities corresponding to the density values of the original can be corrected in a similar manner.

The circuit which has been described includes a (litterence circuit and a non-linear circuit which are somewhat similar to those used in two-stage masking but in apparatus embodying the present invention the gain of these circuits need not be so high because their resultant signal is combined with the correcting channel signal which therefore acts in a manner similar to single-stage masking. Thus, single-stage and two-stage masking are combined to obtain theladvantages of two-stage masking without its disadvantages. In addition, the use of some degree of single-stage masking enable some removal of neutral tones, which is desirable where a black printer is being used.

In order that the invention may be better understood, several embodiments will now be described with reference to the accompanying drawings in which:

FIGURE 1 shows diagrammatically a first form of colour-correction apparatus embodying the invention;

FIGURE 2 illustrates a circuit used in the apparatus of FIGURE 1;

FIGURE 3 shows diagrammatically a further form of apparatus embodying the present invention for colour correction;

FIGURE 4 shows diagrammatically apparatus for producing a corrected black printer, and

FIGURE 5 shows a circuit used in the apparatus of FIGURE 4.

In the apparatus shown in FIGURE 1, the light source in the electro-optical scanner is a cathode ray tube 6, which is provided with deflection coils and time-base circuits which produce a rectangular raster on the face of the tube. The intensity of the moving light spot Which forms the raster is modulated by means of a correcting signal which is applied to the grid of the cathode ray tube by way of the conductor 7. The manner in which this correcting signal is obtained will be described later. As is more fully described in Patent Ser. No. 654,408, the modulating signal which is to be applied to the grid of the cathode ray tube first passes through a gate circuit, not shown in the present drawings, which is controlled by a square-wave generator, so that the light spot on the face of the tube is pulsating in character, the object of this being to ensure that the signals derived from photoelectric devices which receive light from the cathode ray tube are alternating signals, as a result of which the design of the subsequent circuits is greatly simplified. The moving light spot on the face of the tube is focused by means of optical systems, which are not shown in FIGURE 1 and which may include partially silvered mirrors, on to three separation transparencies 8R, 8B and 8G. These are the transparencies obtained by exposure to the coloured original through red, blue and green filters respectively, and they are used in the production of the cyan, yellow and magenta printing plates. As the light spot moves over the face-of the cathode ray tube, it scans over and passes through each of the separation transparencies, the intensity of the light transmitted through each transparency depending upon the transmission factor of the transparency at the point which is being scanned. An unexposed light-transmitting photographic plate or film which is to provide .the yellow printer is placed immediately behind and in contact with the separation transparency 8B, and as a result this plate or film will be exposed in accordance with the information on the blue separation transparency 8B, and also in accordance with the modulation on the face of the cathode ray tube. The plate or film 10 is backed by a filter 12 such that it passes only light to which the plate or film is insensitive. Thus the backing filter 12 prevents undesired exposure of the plate or film due to light to which the plate or film is sensitive reaching the latter through the back surface of the plate or film. Assuming the plate or film to be sensitive only to the blue end of the spectrum, the backing filter absorbs blue light but transmits light in the remainder of the spectrum.

The light which passes through the plate or film and the filter 12 is difiused thereby, and difiusing plates 14R and 14G are placed behind the separation negatives 8R and 8G to diffuse the light which is transmitted through these negatives to the same extent.

Light-integrating units 16R, 16B and 16G collect the light which is transmitted through the three transparencies and direct it on to three photo-multipliers 18R, 18B, and 13G. The signals from the three photo-multipliers vary with the transmission factors of the successively scanned elements of the three separation transparencies. In the apparatus shown in FIGURE 1, with the blue separation transparency in the centre position, the yellow channel signal from the photo-multiplier 18B is first applied to an inverter 20 which reverses the polarity of the signal and applies it to two mixers 22 and 24. In the mixers 22 and 24, the inverted yellow channel signal is added to the cyan channel signal from the photo-multiplier 18R and the magenta channel signal from the photo-multiplier 18G, respectively. The output of each mixer represents the sum of its two input signals, the output of mixer 22 being thus (R-B) and the output of mixer 24 being (GB), in which R, B and G represent the red, blue and green colour channel signals. These two difference signals are applied through gain controls 26 and 28 to non-linear amplifiers in the form of blocking circuits 30 and 32. The controls 26 and 28 serve to adjust the D.C. level of the difierence signals so that the subsequent blocking circuits pass only a controlled portion of the larger positive difference signal, that is to saythe blocking circuit 30 passes signals corresponding to areas in which the cyan component exceeds the yellow component, and the blocking circuit 32 passes signals corresponding to areas in which the magenta component exceeds the yellow component. The resultant signals from the blocking circuits 30 and 32, representing cyan and violet areas, and magenta and violet areas respectively, are added in a mixer circuit 34, which also receives through further gain controls 36 and 38 the output signals from the photo-multipliers 18R and 18G. The mixer circuit 34 provides a final correction signal which is the sum of its four input signals, and this final correction signal is applied by way of the conductor 7 to the grid of the cathode ray tube 6. v

The advantage of this arrangement will be seen from the following. The green separation transparency provides the information for the magenta printer, since the magneta absorbs green but reflects blue and red. Thus for maximum green component in the reproduction, no magenta ink is printed, and for the reproduction of a green tone, no magenta ink is printed, and equal amounts of cyan and yellow are printed, the cyan absorbing the red component of the light and the yellow absorbing the blue component. Thus the green tone is the value produced by printing equal amounts of cyan and yellow, and the correction of the yellow printer by the cyan channel signal would reduce the green scale. In the apparatus shown in FIGURE 1, the signal applied from the blocking circuit 39 to the mixer 34 is present only when the cyan channel signal exceeds the yellow channel signal, that is to say for cyan and violet tones. The mixer 34 receives a signal directly from the photomultiplier 18R through the grain control 36 whenever there is a cyan channel signal, that is to say for all areas of the reproduction which are cyan, violet, green or grey. Thus as a result of the addition of the cyan and violet signal from the blocking circuit 30', the setting of the gain control 36 can be reduced to a value for which a reasonable level of density in green can be maintained.

In a similar manner, red is produced in the reproduction by equal amounts of magenta and yellow, since these inks absorb green and blue respectively, and cyan ink would absorb red. A signal is applied to the mixer 34 from the blocking circuit 32 only for magenta and violet areas of the original, while a signal is applied to the mixer 34 from the photo-multiplier 186 through the gain control 38 for violet, magenta, red and grey areas. Again, as a result of the addition of the signal from the blocking circuit, the setting of the gain control 36 can be reduced to a value for which a reasonable level of density in red may be maintained.

In the apparatus shown diagrammatically in FIGURE 4, the inverter circuit 20 is of conventional design and the mixer circuits 22, 24 and 34 are conventional summing amplifiers.

FIGURE 2 shows the blocking circuit used in FIG- URE 1. The input signal is applied through conductor to the andoe of the diode 52, the cathode of which is connected to a point of fixed potential in a potential divider including the resistors 54 and 56. When the input signal is more positive than the junction of the cathode of the potential divider, an output signal will appear on the conductor 58. When the input signal is negative with respect to this junction, no output signal will appear on the conductor 58.

In the case of the magenta printer it is generally required to correct only for the cyan content of the printer. In this case, however, as shown in FIGURE 3 (in which the light-transmitting printing plate is in the green separation negative or magenta printer channel), the mixer 24 and blocking circuit 32 are omitted. The output of the mixer 22 represents the cyan channel signal minus the magenta channel signal, and the positive portions of this output signal are applied through the blocking circuit to the mixer 34. However it may also be desirable in the case of the correction of the magenta printer to provide compression of the magenta positive contrast as a whole, if the magenta positive is to receive the same photoraphic development as the yellow plate. This is achieved by applying an output signal from the photo-multiplier 18G through the gain control to the mixer 34.

A somewhat similar technique, embodying the apparatus according to the invention, can be used in the correction of a black printer which has been obtained by exposure to white light through the original or by exposure to the original through red, blue and green filters in succession. The correction may, for example, take the form of a reduction of the densities representing cyan and yellow areas of the original. 'Ilhis arrangement is shown in FIGURE 4, in which the black printer negative is placed in the centre channel in front of a light-transmitting printing plate which is to be exposed. The light which passes through this combination is received by the centre photo-multiplier 18BK of FIGURE 4, and the cyan and yellow separation negatives are placed in the outer channels and are scanned to provide output signals from the outer photo-multipliers 18R and 18B which represent the transmission factors of successively scanned elemental areas of the separation negatives produced by means of the red and blue filters. The signals from the photo-multipliers 18R and 18B are applied to a maximum signal selector circuit 62, the output of which corresponds at any instant to the larger of these two input signals. The signal from the centre photo-multiplier 188K is applied through an inverter 64 to a mixer 66 which also receives the output of the maximum signal selector circuit 62. The signal from the mixer, which represents the result of subtracting the black printer signal from the yellow or cyan printer signals, is applied through a gain control 68 to a blocking circuit 70 which passes only the more positive portions of the waveform. The output signals from the blocking circuit are applied to a mixer 72 in which they are added to signals received directly from the photo-multipliers 18R and 183. The mixer output signal is used to modulate the cathode ray tube. It is arranged that the colour signal from the maximum signal selector circuit 62 and the inverted black printer signal, which are added in the aosaasa first mixer in opposite polarities, are of equal amplitudes for grey tones on the original so that they cancel each other out in the mixer 66. In general, the colour signal from the circuit 62 is of greater amplitude than the signal obtained by scanning the black printer negative. It will be seen from this that the signals from the mixer 66 therefore represent colours in the briginal and not greys and are all of one polarity. The blocking circuit serves to reduce the effect of electrical noise in the correction signal. This noise may constitute an appreciable percentage of a difference signal when the latter is small, but the blocking circuit 70 allows only the larger signals, in which the electrical noise has less effect, to pass. The inclusion of the correction channel which comprises the maximum signal selector circuit and the non-linear amplifier permits further reduction of the colour densities in the black printer without further compression of the grey tones, and the amount of the further reduction of the colour densities may be controlled by means of the gain control 68 and the gain controls 74 and 76.

The maximum signal selector circuit is shown in FIG- URE 5, from which it will be seen that the two input signals are applied over the conductors 40 and 42 to the anodes of two diodes 44 and 46, the cathodes of which are connected together and to a point of suitable fixed potential. If the signal on conductor 40 is more positive than the signal on conductor 42, the potential of the cathodes will rise to almost the potential of the conductor 40, and as a result the diode 46 will be blocked. Similarly, if the conductor 42 is at a more positive potential than the conductor 40, the diode 44 will be blocked. In each case the output signal on conductor 48 will correspond to the more positive of the two input signals. 7

Although in the apparatus which has been described the photographic plate or film to be exposed is placed in contact with the corresponding separation negative, and is of such a kind that the scanning light rays pass through the combination to a photo-multiplier, any other form of electro-optical scanner can be used to produce the electrical signals representing the colour components of the original and to expose the photographic emulsion from which the printer is to be prepared. Similarly, other forms of light source can be used in place of the cathode ray tube, and if desired a number of light sources can be used for scanning the separation negatives and the printer positive. In a further alternative, the colourcomponent signal representing the colour to be corrected is added to the signal which, in the example described above, was used to modulate the intensity of the scanning light spot, the corrected colour-component signal thereby obtained being used to modulate a light source which directly exposes the plate or film which will provide the colour printer. Furthermore, the separation negatives can be replaced by the original transparency, the scanning light rays which pass through the original being separated and passed through filters to fall on photoelectric devices which provide colour-component signals.

We claim:

1. Apparatus for the reproduction of coloured originals comprising: electro-optical scanning means for deriving electric colour channel signals representing the colour components of a coloured origin-a1; subtraction means connected to said scanning means to receive a signal representing a given colour component to be corrected and a correcting colour channel signal for providing an output signal representing the difierence between said two signals; a non-linear circuit connected to receive the output signal from said subtraction means said circuit having an input-output characteristic in which the gain is reduced as the input-increases, whereby at least the difference signals of a polarity representing an excess of the signal to be corrected over the correcting colour are attenuated; a light source for exposing a light-sensi tive layer which is to provide the colour printer for the colour to be corrected; and exposure control means for modulating the light source both with the output signal from said non-linear circuit and with said correcting signal from the same correcting colour channel.

2. Apparatus according to claim 1 comprising: further subtraction means connected to said scanning means to receive the signal representing the given colour component to be corrected and a second correcting colour channel signal for providing an output signal representing the difference between said two signals; a second nonlinear circuit connected to receive the output signal from said further subtraction means and having an input-output characteristic in which the gain is reduced as the input increases, whereby at least the difference signals of a polarity representing an excess of the signal to be corrected over the second correcting colour signal are attenuated, and in which said exposure control means also modulates said light source both with the output signal from said second non-linear circuit and with said correcting signal from said second correcting colour channel.

3. Apparatus according to claim 1, wherein said light source comprises a cathode ray tube, and in which the unexposed light-sensitive layer which is to provide a colour printer is exposed through corresponding separation transparency to a rectangular raster on the face of said cathode ray tube, and means for modulating the light spot forming said raster by said exposure control means.

4. Apparatus according to claim 1, in which said electro-optical scanning means is arranged for scanning an uncorrected black printer separation having density values corresponding to the density values of the coloured original to provide an uncorrected black printer signal; subtraction means connected to said scanning means to receive said uncorrected black printer signal and a scanner signal representing a given colour component of the original for providing an output signal representing the difierence between said two signals; said apparatus further comprising a non-linear circuit connected to receive the output signal from said subtraction means and having an input-output characteristic in which the gain is reduced as the input increases; and further exposure control means, for modulating said light source both with the output of said last-named non-linear circuit and said last-named colour-component signal, in order to expose a light-sensitive layer which is to provide the black printer.

5. Apparatus according to claim 4, including a maximum signal selector circuit connected to receive said colour component signal and at least one other colourcomponent signal and connected for applying the larger of the said colour-component signals to the said subtraction means, said further exposure control means being connected to said scanner for additionally modulating said light source with said other colour-component signal.

References Cited in the file of this patent UNITED STATES PATENTS 2,873,312 Moe Feb. 10, 1959 

1. APPARATUS FOR THE REPRODUCTION OF COLOURED ORIGINALS COMPRISING: ELECTRO-OPTICAL SCANNING MEANS FOR DERIVING ELECTRIC COLOUR CHANNEL SIGNALS REPRESENTING THE COLOUR COMPONENTS OF A COLOURED ORIGINAL; SUBTRACTIONN MEANS CONNECTED TO SAID SCANNING MEANS TO RECEIVE A SIGNAL REPRESENTING A GIVEN COLOUR COMPONENT TO BE CORRECTED AND A CORRECTING COLOUR CHANNEL SIGNAL FOR PROVIDING AN OUTPUT SIGNAL REPRESENTING THE DIFFERENCE BETWEEN SAID TWO SIGNALS; A NON-LINEAR CIRCUIT CONNECTED TO RECEIVE THE OUTPUT SIGNAL FROM SAID SUBTRACTION MEANS SAID CIRCUIT HAVING AN INPUT-OUTPUT CHARACTERISTIC IN WHICH THE GAIN IS REDUCED AS THE INPUT INCREASES, WHEREBY AT LEAST THE DIFFERENCE SIGNALS OF A POLARITY REPRESENTING AN EXCESS OF THE SIGNAL TO BE CORRECTED OVER THE CORRECTING COLOUR ARE ATTENUATED; A LIGHT SOURCE FOR EXPOSING A LIGHT-SENSITIVE LAYER WHICH IS TO PROVIDE THE COLOUR PRINTER FOR THE COLOUR TO BE CORRECTED; AND EXPOSURE CONTROL MEANS FOR 